This invention relates to radar jamming systems and more particularly to the use of a primed oscillator in combination with an antenna and a keying circuit as an expendable decoy ejected from a target which produces suitable jamming signals as the decoy moves away from the target.
One of the frequently occurring situations necessitating radar jamming is a situation involving an incoming missile which homes on a xe2x80x9ctargetxe2x80x9d via a CW radar signal. This signal, when returned from a xe2x80x9ctargetxe2x80x9d, is analyzed as to doppler frequency shift which provides the relative velocity of the xe2x80x9ctargetxe2x80x9d with respect to the missile, and as to modulation which provides the angular bearing of the xe2x80x9ctargetxe2x80x9d. In the past, jammers for this tactical situation have involved the generation of a CW signal by the target in which the frequency of the signal is made to vary in a predetermined fashion to convey false doppler frequency shift information. This is commonly called xe2x80x9cstealing the speed gatexe2x80x9d of the xe2x80x9cenemyxe2x80x9d radar. Additionally, the jamming signal is modulated to convey false angular bearing information. In the prior art jamming devices, it is usually necessary to detect the frequency of the incoming radar signal and to synthetically produce a signal having this frequency. Having generated this signal, the frequency of this signal is slowly varied for the required doppler shift and is modulated to give the false bearing information.
By way of background, there have been basically three approaches to generating the above described CW jamming. The first is a CW amplification approach in which the incoming signal is amplified by a chain of amplifiers and reradiated. In this approach, the false doppler and bearing modulations are applied during amplification and two antennas are required. This approach suffers from two disadvantages. 1) A very high antenna isolation is required to prevent a ring-around type feedback between the antennas. On many installations sufficient isolation is physically impossible to achieve. 2) A separate CW power amplifier tube is required, in addition to the pulse tube which is normally carried for pulse jamming.
The second approach eliminates the antenna isolation problem by using an oscillator to generate the jamming signal. The oscillator is set on frequency by a receiver which briefly samples the received radar signal at periodic intervals. Thus it will be appreciated that the jamming must be interrupted during these xe2x80x9clook-thru""sxe2x80x9d, so that received radar signals may be sampled. This approach suffers from the disadvantage that the receiver for the jammer is either complex or has a slow response. In addition, a separate CW power tube is still needed.
The third approach makes use of the pulsed power amplifier and simulates CW by a train of pulse bursts. The switching-off between pulses solves the aforementioned ring-around problem, and the time between bursts is available for pulse jamming other threats. However, this approach suffers a severe power/efficiency limitation because the effective power is reduced from the average power by the duty factor. By way of example, typical pulse tubes are limited to a duty factor of about 10%; therefore the effective power is only about 1% of the peak power. It will be appreciated that all of these jammers are carried xe2x80x9con boardxe2x80x9d the xe2x80x9ctargetxe2x80x9d.
Instead of xe2x80x9con-boardxe2x80x9d apparatus, doppler jamming may be accomplished by, for instance, the ejecting of a repeater or transponder at an angle from an aircraft. It is the function of the repeater or transponder to duplicate the incoming signal, with false doppler information being introduced by the different velocity of the ejected decoy with respect to the xe2x80x9cenemyxe2x80x9d radar, and with the false bearing information being automatically provided by virtue of the angular difference between the ejected decoy and the true xe2x80x9ctargetxe2x80x9d.
In the subject invention, an expendable jammer is ejected as a decoy from a target for countermeasuring CW signals from xe2x80x9cenemyxe2x80x9d radars, in which the jammer includes a single port, keyed, high power oscillator which is primed with the low level signal received from the xe2x80x9cenemyxe2x80x9d radar. The priming signal is injected directly into the tank circuit of the oscillator which is rapidly turned on and off (keyed) to produce a rapidly pulsed signal which tracks the frequency of the incoming signal. The keying is accomplished either by removing the power from the oscillator, by shorting the tank circuit, or by gating the negative resistance element in the oscillator.
It is important to the understanding of the subject system to distinguish the subject priming system from traditional injection locking systems. In injection locking the injected signal is a relatively high level signal strong enough to lock the oscillator frequency to the injected frequency. The signal from the xe2x80x9cenemyxe2x80x9d radar normally does not reach injection locking levels at the xe2x80x9ctargetxe2x80x9d and thus injection of a received signal into a remote oscillator circuit will not lock the remote oscillator to the frequency of the received signals. In priming, the injected signal need only be strong enough to force the oscillator to start up in phase with the priming signal at the start of each pulse. In priming there is no locking or changing of the oscillator. Rather the oscillator is allowed to run at its natural frequency and is rapidly keyed so that the oscillator is turned off before its signal can accumulate a significant phase error with respect to the incoming signal. The result is that the output signal from the oscillator tracks the frequency of the priming signal to the extent that the phase difference between the priming signal and the natural frequency signal from the oscillator is small. Since this phase difference can be made very small by rapid keying, the frequency of the output signal from the keyed primed oscillator approximates or tracks that of the priming signal.
The resultant signal from the oscillator is coupled back to the same antenna used for receiving the xe2x80x9cenemyxe2x80x9d radar signal and is transmitted back to the receiving section of the xe2x80x9cenemyxe2x80x9d radar which cannot distinguish the individual pulses of returned energy due to the high PRF (pulse repetition frequency) of the returned signal. Thus, the signal from the jammer is detected as a CW signal. By virtue of ejecting the jammer, the enemy radar is given false bearing and doppler information causing the enemy radar to either lose track or to home on the expendable jammer. The expendable jammer also has a multiple threat capability in which simultaneously arriving signals from different xe2x80x9cenemyxe2x80x9d radars result in a jammer output at each of the different frequencies of the incoming signals with sufficient jamming power at each frequency.
The primed oscillator is therefore a very simple broadband jammer which can follow the incoming frequency by virtue of the rapid keying or chopping. This rapid keying or chopping while permitting the oscillator to follow the frequency of the incoming signal also results in a pulsed signal with a sufficiently high PRF that the xe2x80x9cenemyxe2x80x9d radar receiver cannot resolve the pulses of the returned signal and thus xe2x80x9cseesxe2x80x9d only a CW signal. Due to the availability of IMPATT diode oscillators which have outputs exceeding 100 watts, amplification stages are unnecessary. Moreover since the duty cycle can be close to 100%, the effective power of the oscillator can be very high. Since the subject system utilizes only one antenna there is no isolation problem. It is also an important feature of the primed oscillator that it can be used in its chopped or keyed mode for handling simultaneous multiple incoming signals. This comes about as follows. When two or more CW signals are present, at different frequencies, their resultant is a single signal of varying phase and amplitude. Each time the oscillator is keyed on it is primed by that resultant. Its phase on successive pulses will therefore faithfully follow the phase of the resultant signal, thereby in effect reproducing all the incoming frequencies.
Thus the use of a primed oscillator introduces simplicity and low weight due to the lack of amplification stages necessary, and the power supply requirements are minimized due to the fact that the oscillator signal is utilized directly rather than being again amplified. When this jammer is ejected from a potential xe2x80x9ctargetxe2x80x9d no additional circuitry is required for false doppler information because the movement of the decoy away from the target provides false bearing information automatically, while the speed difference between the true xe2x80x9ctargetxe2x80x9d and the decoy relative to the incoming missile due to drag on the decoy provides false doppler information.
It is therefore an object of this invention to provide a system for jamming in which a primed oscillator is utilized in a rapidly keyed mode to generate a rapidly pulsed jamming signal having an apparent frequency which duplicates that of the incoming signal.
It is another object of this invention to provide a expendable jammer utilizing a rapidly keyed primed oscillator in which the jammer package is ejected from a xe2x80x9ctargetxe2x80x9d in such a manner that the apparent angular bearing and relative speed between the xe2x80x9cenemy radarxe2x80x9d and the xe2x80x9ctargetxe2x80x9d are effectively altered.
It is a still further object of this invention to provide a jamming system in which amplifiers or amplification stages are eliminated in favor of a high power primed oscillator which is rapidly keyed to provide an essentially continuous CW signal made up of numerous packets of signals, each of which is initially in phase with the incoming radar signal such that the output signal from the jamming system has a synthetically produced frequency approximating that of the priming signal.
It is another object of this invention to provide a jamming system utilizing a rapidly keyed primed oscillator having a multi-threat capability in which the simultaneous arrival of two or more different signals from different xe2x80x9cenemyxe2x80x9d radars results in the production of a signal having frequency components which duplicate those of the incoming signals. BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1C are diagrams and diagramatic representations of the subject keyed, single-port primed oscillator in which an incoming signal is coupled into the tank circuit of the oscillator;
FIG. 2 is a waveform diagram illustrating the operation of the primed oscillator with and without priming signals;
FIG. 3 is a waveform diagram illustrating the operation of the primed oscillator in the time domain with respect to the increasing phase error during the transmitted pulse and also the effective amplitude in the receiver of the xe2x80x9cenemyxe2x80x9d radar with respect to this phase error;
FIG. 4 is a diagramatic representation of the multi-threat capability of the primed oscillator illustrating the case in which simultaneous signals of different frequencies but of equal amplitudes arrive at the jammer;
FIG. 5 is a diagramatic representation of the multi-threat case in which signals of different frequencies and unequal amplitudes are simultaneously present at the jammer;
FIG. 6 is a sectional and schematic diagram of an IMPATT diode oscillator for use with the subject system;
FIG. 7 is a schematic diagram of the IMPATT diode oscillator of FIG. 6;
FIG. 8 is a low frequency analog of the IMPATT diode oscillator of FIG. 7; and
FIG. 9 is a schematic diagram of the keyed bias supply of FIG. 6.